1. Field of the Invention
The present invention relates to a method for evaluating the quality of a digital signal that has been read out from a storage medium and then decoded by a maximum likelihood decoding technique, and also relates to an apparatus for reading information from a storage medium and performing such quality evaluation on the read signal.
2. Description of the Related Art
Recently, various types of apparatuses (such as hard disk drive (HDD), optical disk drive and magneto-optical disk drive) for reading digital information from a storage medium have been used extensively in audiovisual appliances, personal computers and so on.
FIG. 1 is a block diagram showing a configuration for a part of a conventional optical disk drive 900. A light beam that has been reflected from an optical disk 1 is converted by an optical head 2 into a read signal. The read signal has its waveform shaped by a waveform equalizer 3 and then digitized by a comparator 4. The threshold value of the comparator 4 is normally subjected to a feedback control so that the output digital signals of the comparator 4 equals zero when integrated together.
In the optical disk drive 900, normally a phase-locked loop (PLL) circuit generates a clock signal that is synchronized with a read signal. A clock signal of that type is termed “a read clock signal”. As shown in FIG. 1, the PLL circuit includes a phase detector 5, a low-pass filter (LPF) 6 and a voltage controlled oscillator (VCO) 7. To generate the read clock signal, the phase detector 5 detects a difference in phase between the output digital signal of the comparator 4 and an output clock signal of the VCO 7. The phase difference detected is averaged by the LPF 6. In accordance with the output of the LPF 6, the control voltage of the VCO 7 is determined. In this manner, the oscillation frequency of the VCO 7 is subjected to a feedback control so that the phase difference output from the phase detector 5 always equals zero. Thus, the VCO 7 can output a clock signal that is synchronized with the read signal. By using a PLL circuit like this, even if the disk has some degree of eccentricity, for example, a clock signal can be extracted constantly so as to be synchronized with the read signal.
The read clock signal is used to determine whether the recorded code (i.e., digital information) is one or zero. More specifically, the digital information can be read out by determining whether or not each detection pulse of the comparator 4 falls within a window width defined by the read clock signal. As used herein, the “detection pulse” of the comparator 4 refers to a portion of the output digital signal of the comparator 4 that exceeds the predetermined threshold value.
However, the output detection pulse of the comparator 4 might deviate from the window width of the read clock signal due to various factors including intersymbol interference occurring in the read signal, the distortion of a recording mark, circuit noise and a control residual of the PLL. In that case, an error occurs. Such a time lag created between the detection pulse of the comparator 4 and the read clock signal is called a “jitter”.
In reading digital information by the technique described above, the quality (which is represented in terms of an error rate) of the read signal can be evaluated by using the distribution of jitter. The jitter distribution may be supposed to form a normal distribution having a mean of zero. In that case, the error rate Pj (σ/Tw) is given by
                              Pj          ⁡                      (                          σ              /              Tw                        )                          =                  2          ⁢                                          ⁢                      erfc            (                                          Tw                /                2                            σ                        )                                              (        1        )                                          erfc          ⁡                      (            z            )                          =                              1                                          2                ⁢                                                                  ⁢                π                                              ⁢                                    ∫              z              ∞                        ⁢                                          exp                ⁡                                  (                                      -                                                                  u                        2                                            2                                                        )                                            ⁢                                                          ⁢                              ⅆ                u                                                                        (        2        )            where σ is the standard deviation of the jitter distribution that is supposed to be a normal distribution and Tw is the window width.
FIG. 2 is a graph showing a relationship between the jitter and the bit error rate (BER). As can be seen from FIG. 2, as the standard deviation of the jitter increases, the BER also increases. The jitter of a read signal can be actually measured with a time interval analyzer (TIA). Accordingly, even if no errors have actually occurred, the quality of the signal can also be evaluated by the jitter standard deviation σ per the window width Tw. Thus, it is possible to predict the probability of occurrence of errors (which will be herein referred to as an “error probability”). For that reason, by measuring the standard deviation of the jitter, the performance of a given drive, a storage medium or an optical head can be checked and tested. Also, if the parameters of an equalizer are controlled in such a manner as to decrease the standard deviation of the jitter, then a read operation can be performed even more constantly.
In the technique described above, digital information is directly obtained from the output digital signal of the comparator 4. According to another known technique on the other hand, digital information may also be obtained by a maximum likelihood decoding method. Examples of known maximum likelihood decoding methods include a partial response maximum likelihood (PRML) method. In the PRML method, data is read or written from/on a storage medium having a high storage capacity with the potential occurrence of intersymbol interference fully taken into account. More specifically, a signal that has been read out from such a high-capacity storage medium is subjected, by a waveform equalizer, a digital filter and so on, to a partial response equalization so as to have a predetermined frequency characteristic. Then, the PR equalized and filtered signal is decoded into most likely (or most probable) digital data by a Viterbi decoding technique, for example. According to the PRML method, data can be decoded at a low error rate even from a read signal with a low signal-to-noise ratio (SNR) or a read signal that is affected by the intersymbol interference relatively seriously.
In a maximum likelihood decoding method like this, data is decoded from a read signal by selecting a most probable state transition path. In general, a quantity representing the probability of a state transition that leads to a state Sn (where n is a state number) at a time k is defined by the following Equation (3):
                              L          Sn                =                              ∑                          i              =              0                        k                    ⁢                                    (                                                y                  i                                -                                  level                  v                                            )                        2                                              (        3        )            where yi is the actual value of the read signal (or digital sample data) at a time i and level, is an expected ideal value of the read signal.
In a maximum likelihood decoding method, a state transition path having the minimum probability quantity as represented by Equation (3) is selected. Unlike the above-described technique of decoding the data as one or zero by determining whether or not the detection pulse falls within the window width at each point in time k, a Euclidean distance of (yk−levelv)2 is obtained from the data that is sampled at each point in time k by reference to a read clock signal according to the maximum likelihood decoding method. Then, the data is decoded based on the Euclidean distance. Accordingly, the decoded result obtained by the maximum likelihood decoding method is also affected by a past sampled value yk of a read signal.
In this maximum likelihood decoding method, even when two read signals have the same jitter standard deviation σ, errors may or may not have occurred in the read signals. For that reason, it is difficult to estimate the error rate of the decoded digital data, obtained by the maximum likelihood decoding method, by the jitter standard deviation σ of the read signal. Accordingly, an error rate estimating method (i.e., a signal quality evaluating method), which is more suitable to the maximum likelihood decoding method, needs to be used.
A method for evaluating the quality of a signal that has been decoded by the maximum likelihood decoding method is disclosed in Japanese Laid-Open Publication No. 10-21651, for example. The apparatus disclosed in Japanese Laid-Open Publication No. 10-21651 obtains a difference in likelihood between two state transition paths, having a minimum Euclidean distance between them, and then processes this difference by a statistical method, thereby evaluating the quality of the signal.
More specifically, to obtain a difference in likelihood between two paths that result in the same state at a time k, the sums of branch metrics of two survived paths that were regarded as most likely for two mutually different states at the previous time k−1 are used. However, these sums of branch metrics at the time k−1 might be those of unwanted paths. For example, a path other than the path in question (i.e., a path having likelihood to be checked) may have been selected by mistake before the time k−1. Japanese Laid-Open Publication No. 10-21651 does disclose a technique of selecting two paths having the minimum Euclidean distance between them and obtaining a difference in likelihood between these two paths. However, Japanese Laid-Open Publication No. 10-21651 does not disclose any specific method for calculating the target likelihood values of these two paths with more certainty.